ITPEC IP April 2023 Question 28
Which of the following is a term for an operation that extracts only the rows (records) with a price of 100 yen or more, from the “Product” table of a relational database?
Product
Explanation
Selection — the relational operation that extracts the rows satisfying a condition, here a price of 100 yen or more.
Applying the condition to the Product table keeps three of the five rows, with every column:
Pencil (50) and Eraser (80) fall below 100 and are dropped.
Answer (c)
Why not others:
- •(a) Join — combines two tables into one by matching a common column; only one table is involved here
- •(b) Projection — extracts columns, for example only Product_name and Price, and keeps every row
- •(d) Union — combines the rows of two tables with the same structure into one table
Key rule: Selection cuts a table horizontally (rows), projection vertically (columns); join puts tables side by side, union stacks tables of the same structure.
Databases · AI-written, may contain errors
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